Adaptive energy management for vertical speed control of an aircraft

ABSTRACT

A system for generating an optimum vertical speed command and controlling the aircraft to this command by allowing the aircraft to assume an optimum trimmed condition for the thrust energy available which system utilizes the aircraft autopilot system.

This invention relates to vertical speed control of an aircraft and moreparticularly to adaptive energy management for vertical speed control ofthe aircraft.

Present systems for generating the go-around command utilize the conceptdisclosed in U.S. Pat. No. 3,847,328 issued Nov. 12, 1974, or speedcommand/angle of attack computers. The concept disclosed inaforementioned U.S. Pat. No. 3,847,328 commands a fixed vertical speedwhile the speed command/angle of attack concept is complex and involvesthe use of separate computers and sensors to perform the go-aroundmaneuver.

It is accordingly an object of this invention to provide a systemutilizable for vertical speed control including, e.g., go-around controlwhich commands an optimum vertical speed for the available thrust, andis also fail safe (uses circuitry known to be operating during theapproach) and further, has the capability of compensating for enginefailures or thrust mismanagement.

The foregoing and other objects and advantages of the invention willappear more fully hereinafter from a consideration of the detaileddescription which follows taken together with the accompanying drawingswherein an embodiment of the invention is illustrated by way of example.

In the drawings:

FIG. 1 is a block diagram illustrative of an automatic approach andgo-around mode as shown in the aforementioned U.S. Pat. No. 3,847,328;

FIG. 2 is a block diagram illustrative of speed command/angle of attackcomputers for generating a go-around command;

FIG. 3 is a block diagram illustrative of the system concept of thepresent invention; and,

FIG. 4 is a detailed block diagram of the system of FIG. 1 utilizing theconcepts shown in FIG. 3.

Turning now to FIG. 1 showing the system, a state of the art system willbe recognized which is simple and fail safe (utilizes circuitry known tobe operating during the approach) but always commands a fixed verticalspeed which is not necessarily optimum for the thrust available.

Briefly now turning to FIG. 2, it will be observed that the speedcommand/angle of attack concept shown therein is complex since itinvolves the use of separate computers and sensors to perform thego-around maneuver. It should be further noted that such system conceptis not fail safe in that it could produce a nose down hardover maneuverat critical altitude (when a go-around is initiated) although itincludes the capability of handling engine failures or thrustmismanagement.

A comparison now of FIGS. 1 and 4 will show that the fixed maximumvertical speed bias and the flaps <23° switch of FIG. 1 have beenreplaced by a variable vertical speed command proportional to the changein stabilizer position during the go-around of the type shown in FIG. 3.

Turning now to the detailed description of the system embodiment shownin FIG. 4 illustrative of the present adaptive energy management systemfor generating an optimum vertical speed command for the availablethrust energy level and also controlling the aircraft to this command byallowing the aircraft to assume an optimum trimmed condition, it will benoted that the system is particularly useful for but not limited to usein the aircraft go-around condition.

In operation of the system, prior to go-around, a feedback signaldenoting a delta stabilizer position signal Δδ_(s) is zero and the totalvertical speed command (h_(c)) is initially equal to an initial verticalspeed bias signal h_(IB) and a variable vertical speed command h_(VB)passed through a delay circuit. The h_(VB) signal is obtained from aunit whose input is Δδ_(s) and whose operand is ##EQU1## . The autopilotsystem further compares h_(c) with an altitude rate signal h to derivean h_(e) signal which is fed into a network having a transfer functionKδ_(e) /h_(e) to produce an elevator command signal δ_(ec). The lattertogether with a damping signal and an elevator feedback signal arecompared to produce an elevator control signal. At the start of ago-around the aircraft assumes the initial vertial speed h_(IB) with atime constant τ of the delay network. If the available thrust level isgreater than that required to maintain h_(IB) an increase in speed willoccur which results in additional lift and a nose down elevator commandgenerated by the autopilot system so as to retrim the aircraft. Thelatter elevator command δ_(e) is inputted into an automatic stabilizertrim unit which repositions the stabilizer to produce a stabilizersignal δ_(s) and thereby returns the elevator to trim neutral. Thestabilizer signal is inputted into a Delta Stabilizer Position Sensor todevelop the aforementioned delta stabilizer position signal Δδ_(s). Thelatter signal is indicative of the excess energy within the system andserves to modulate the vertical speed command signal h_(VB) causing theaircraft to increase its rate of climb. On the other hand, if thevertical speed command h_(c) is greater than the available thrust level,the aircraft will lose speed and lift causing the autopilot to provide anose up elevator signal and the stabilizer will be repositioned in anose up direction. This results in Δδ_(s) which reduces the verticalspeed command h_(c).

It can be seen from the followng that the principles of operation aresimple, yet extremely effective. Referring still to FIG. 4, thefollowing is a summary in brief of system operation:

1. Prior to go-around the Δδ_(s) position signal is zero and the totalvertical speed command (h_(c)) initially is equal to h_(IB).

2. at go-around initiation, the airplane will assume the initialvertical speed h_(IB) with the constant τ.

3. If the thrust level available during the go-around maneuver is inexcess of that required to maintain h_(IB), then the airplane will beginto increase in speed. This increase in speed generates additional liftand results in the autopilot commanding a steady state nose downelevator (δ_(e)) to retrim the airplane.

4. The automatic stabilizer trim unit will reposition the stabilizer(δ_(s)) returning the elevator (δ_(e)) to trim neutral and producing adelta stabilizer position (Δδ_(s)) signal.

5. This signal (Δδ_(s)) is representative of excess energy within thesystem and is used to modulate the vertical speed command (h_(VB))causing the airplane to increase its rate of climb.

6. Conversely, if the vertical speed command is in excess of the climbcapability of the airplane due to thrust limitations, then the airplanewill begin to lose speed and lift. The autopilot will command a nose upelevator (δ_(e)) and the stabilizer (δ_(s)) will be repositioned in anose up direction.

7. This results in a Δδ_(s) signal which reduces the vertical speedcommand (h_(c)).

From the preceding system description including mode of operation it canreadily be seen that the net effect of the system is to provide arelatively simple means of balancing the energy vectors generated in theaircraft to provide an optimum vertical speed for the available thrust.In essence this is accomplished by allowing the aircraft/autopilotsystem to be its own angle of attack computer. The system herein abovedescribed is especially useful in (1) low cost aircraft where theadditional cost of a speed command system would be over-burdensomecostwise or (2) for fail-operative systems where reliability and safetyare the overriding consideration as well as the necessity for lowaltitude go-around capability.

What is claimed is:
 1. In combination in an aircraft flight controlsystem having means for processing signals (h_(VB)) representative ofmaximum vertical speed bias, the improvement comprising:feedback circuitmeans including delta stabilizer position sensing means responsive to astabilizer position of an aircraft for providing a delta stabilizerposition signal Δδ_(s), said feedback circuit means further having anoperand Kh_(c) /Δδ_(s) for providing said signals (h_(VB))representative of maximum vertical speed bias.